Generally because you have opted to have you outgoing mail scanned and your antivirus does not use a recognized encryption certificate. Although in this instance it may be because you are using a hosted server without an encryption certificate for the domain.

What do you see about the server in the certificate when you view it? Issued by your antivirus? or for another server entirely that is related to the hosting service that you trust. You are seeing it because there is an anomaly and the software wants you to make a decision about what is to be done.

Gene ??

I wonder what AV Gene is using?

Might also be causing https://support.mozilla.org/en-US/questions/1514107